Sync createPattern with the spec

third_party/WebKit/Source/modules/canvas2d/CanvasRenderingContext2D.idl

 /*
  * Copyright (C) 2006 Apple Computer, Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 51 matching lines @@
     // image smoothing
     [ImplementedAs=imageSmoothingEnabled, DeprecateAs=PrefixedImageSmoothingEnabled] attribute boolean webkitImageSmoothingEnabled;
     [MeasureAs=UnprefixedImageSmoothingEnabled] attribute boolean imageSmoothingEnabled; // (default True)
     [RuntimeEnabled=ExperimentalCanvasFeatures] attribute ImageSmoothingQuality imageSmoothingQuality; // (default "low")
 
     // colors and styles (see also the CanvasDrawingStyles interface)
     attribute (DOMString or CanvasGradient or CanvasPattern) strokeStyle; // (default black)
     attribute (DOMString or CanvasGradient or CanvasPattern) fillStyle; // (default black)
     CanvasGradient createLinearGradient(double x0, double y0, double x1, double y1);
     [RaisesException] CanvasGradient createRadialGradient(double x0, double y0, double r0, double x1, double y1, double r1);
-    [RaisesException] CanvasPattern createPattern(CanvasImageSource image, DOMString? repetitionType);
+    [RaisesException] CanvasPattern? createPattern(CanvasImageSource image, [TreatNullAs=NullString] DOMString repetitionType);
 
     // shadows
     attribute unrestricted double shadowOffsetX;
     attribute unrestricted double shadowOffsetY;
     attribute unrestricted double shadowBlur;
     attribute DOMString shadowColor;
 
     // rects
     void clearRect(unrestricted double x, unrestricted double y, unrestricted double width, unrestricted double height);
     void fillRect(unrestricted double x, unrestricted double y, unrestricted double width, unrestricted double height);
@@ skipping 59 matching lines @@
 
     // text
     attribute DOMString font; // (default 10px sans-serif)
     attribute DOMString textAlign; // "start", "end", "left", "right", "center" (default: "start")
     attribute DOMString textBaseline; // "top", "hanging", "middle", "alphabetic", "ideographic", "bottom" (default: "alphabetic")
     [RuntimeEnabled=ExperimentalCanvasFeatures] attribute DOMString direction; // "inherit", "rtl", "ltr" (default: "inherit")
 
 };
 
 CanvasRenderingContext2D implements CanvasPathMethods;